Starting in mid-March 2020, many states issued stay-at-home orders or encouraged people to work from home to try to curb the spread of COVID-19. In cities nationwide, daily traffic dropped.

But did fewer cars on the road lead to cleaner air? Not necessarily, according to a new study led by University of Washington researchers.

Using air quality data from U.S. Environmental Protection Agency monitors across the U.S., the research team looked for changes in two common pollutants: ozone, and fine particulate matter called PM2.5. Compared to the past 10 years, neither pollutant has been consistently lower than expected levels since stay-at-home orders began. But the team found that another pollutant that mainly comes from car exhaust, NO2, was at much lower levels in three cities — Seattle, Los Angeles and New York — (30% lower on average) after stay-at-home orders were implemented.

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